1. Field of Invention
The present invention relates to a transmission system for a four wheel drive hybrid electric vehicle, and more particularly, to a transmission system for a four wheel drive hybrid electric vehicle capable of maximizing improvement of fuel efficiency by performing driving of a power split mode having better low-speed driving efficiency and a parallel mode having better high-speed driving efficiency.
2. Description of Related Art
Generally, a hybrid electric vehicle is a vehicle which is driven by effectively combining at least two different kinds of power sources.
The hybrid electric vehicle generally uses an engine and a motor/generator, and uses the motor/generator having a relatively better low-speed torque characteristic as a main power source in a low speed and uses the engine having a relatively better high-speed torque characteristic as the main power source in a high speed.
Therefore, since the hybrid electric vehicle stops an operation of the engine using fossil fuel and uses the motor/generator in a low-speed section, the hybrid electric vehicle has an excellent effect in improvement of fuel efficiency and reduction of exhaust gas.
Further, a transmission system for the hybrid electric vehicle as described above is classified into a single mode type and a multi mode type.
The single mode type may not require connection units, such as a clutch and a brake for transmission control, but may have the reduced fuel efficiency at the time of high-speed driving and require an additional torque multiplication apparatus so as to apply to a large vehicle.
The multi mode type may have increased efficiency at the time of high-speed driving and may be designed to enable torque multiplication, thereby being applied to a medium-large size vehicle.
Recently, the multi mode type is mainly used rather than using the single mode type. Therefore, a research for the multi mode type has been actively conducted.
The transmission system in the multi mode type is configured to include a planetary gear set, a plurality of motors/generators which are used as a motor and a generator, a connection unit which controls rotating elements of the planetary gear set, a battery which is used as a power source for the motor/generator, and the like.
The transmission system in the multi mode type has different operation mechanisms depending on a connection configuration of the planetary gear set, the motor/generator, and the connection unit.
Further, since the transmission system in the multi mode type has characteristics of which durability, power transmission efficiency, a size, and the like vary depending on the connection configuration, a research and development for implementing a power transmission mechanism which is more robust, reduces power loss, and is compact in a field of the transmission system for a hybrid electric vehicle has continued.
The hybrid electric vehicle may be classified into a two wheel drive (hereinafter, referred to as 2WD) type and a four wheel drive (hereinafter, referred to as 4WD) type depending on a driving type.
The 2WD hybrid electric vehicle is generally configured to drive a front wheel using two motors/generators and a planetary gear set.
The 4WD hybrid electric vehicle is configured to drive the front wheel using the two motors/generators and the planetary gear set and is configured to drive a rear wheel using one motor/generator.
However, the 4WD hybrid electric vehicle according to various embodiments of the present invention uses the two motors/generators and the planetary gear set to drive the front wheel to convert a part of the power of the engine into electrical energy at all times, such that fuel efficiency may be reduced in a high-speed region.
Therefore, in order to supplement the above problem, there is a need to increase the number of planetary gear sets and the number of clutches and brakes, which may lead to reduced power transmission efficiency and increased production cost.
The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.